Add a multithreaded test for SIGSTOP and SIGCONT behaviour.

This actually installs a SIGTSTP handler which then triggers a SIGSTOP.
Currently unlinked since both SIGSTOP and SIGCONT delivery is busted.

regress/sys/kern/signal/sig-stop2/Makefile

@@ -0,0 +1,7 @@
+#	$OpenBSD: Makefile,v 1.1 2024/10/07 14:01:12 claudio Exp $
+
+PROG=	sig-stop2
+
+LDADD= -lpthread
+
+.include <bsd.regress.mk>

regress/sys/kern/signal/sig-stop2/sig-stop2.c

@@ -0,0 +1,137 @@
+/*	$OpenBSD: sig-stop2.c,v 1.1 2024/10/07 14:01:12 claudio Exp $	*/
+/*
+ *	Written by Artur Grabowski <art@openbsd.org> 2007 Public Domain.
+ *	Written by Claudio Jeker <claudio@openbsd.org> 2024 Public Domain.
+ */
+#include <sys/types.h>
+#include <sys/time.h>
+#include <sys/wait.h>
+
+#include <err.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <time.h>
+#include <err.h>
+#include <signal.h>
+#include <pthread.h>
+
+#define	THREAD_COUNT	4
+
+volatile sig_atomic_t tstp_count, cont_count;
+pid_t child;
+
+static void
+tstp_handler(int sig)
+{
+	tstp_count++;
+	dprintf(STDERR_FILENO, "SIGTSTP\n");
+	kill(getpid(), SIGSTOP);
+}
+
+static void
+cont_handler(int sig)
+{
+	dprintf(STDERR_FILENO, "SIGCONT\n");
+	cont_count++;
+}
+
+static void
+alrm_handler(int sig)
+{
+	kill(child, SIGKILL);
+	dprintf(STDERR_FILENO, "timeout\n");
+	_exit(2);
+}
+
+
+static void *
+thread(void *arg)
+{
+	struct timespec ts = { .tv_sec = 2 };
+
+	while (nanosleep(&ts, &ts) != 0)
+		;
+
+	return NULL;
+}
+
+static int
+child_main(void)
+{
+	pthread_t self, pthread[THREAD_COUNT];
+	sigset_t set;
+	int i, r;
+
+	signal(SIGTSTP, tstp_handler);
+	signal(SIGCONT, cont_handler);
+
+	sigemptyset(&set);
+	sigaddset(&set, SIGTSTP);
+	sigaddset(&set, SIGCONT);
+
+	self = pthread_self();
+	for (i = 0; i < THREAD_COUNT; i++) {
+		if ((r = pthread_create(&pthread[i], NULL, thread, NULL))) {
+			warnc(r, "could not create thread");
+			pthread[i] = self;
+		}
+	}
+
+	sigprocmask(SIG_BLOCK, &set, NULL);
+
+	for (i = 0; i < THREAD_COUNT; i++) {
+		if (!pthread_equal(pthread[i], self) &&
+		    (r = pthread_join(pthread[i], NULL)))
+			warnc(r, "could not join thread");
+	}
+
+	printf("#tstp = %d #cont = %d\n", tstp_count, cont_count);
+
+	return !(tstp_count == 1 && cont_count == 1);
+}
+
+int
+main(int argc, char **argv)
+{
+	struct timespec ts = { .tv_nsec = 200 * 1000 * 1000 };
+	int status;
+
+	switch((child = fork())) {
+	case -1:
+		err(1, "fork");
+	case 0:
+		exit(child_main());
+	default:
+		break;
+	}
+
+	signal(SIGALRM, alrm_handler);
+	alarm(5);
+
+	nanosleep(&ts, NULL);
+
+	if (kill(child, SIGTSTP) == -1)
+		err(1, "kill");
+
+	if (waitpid(child, &status, WCONTINUED|WUNTRACED) <= 0)
+		err(1, "waitpid");
+
+	nanosleep(&ts, NULL);
+
+	if (kill(child, SIGCONT) == -1)
+		err(1, "kill");
+
+	if (waitpid(child, &status, WCONTINUED|WUNTRACED) <= 0)
+		err(1, "waitpid");
+
+	nanosleep(&ts, NULL);
+
+	if (waitpid(child, &status, 0) <= 0)
+		err(1, "waitpid");
+
+	if (!WIFEXITED(status))
+		err(1, "bad status: %d", status);
+
+	return WEXITSTATUS(status);
+}